In the long term evolution (abbreviated as LTE) system, the user equipment (abbreviated as UE) needs to be uplink and downlink synchronized with the E-UTRAN Node B (abbreviated as eNB) before sending uplink data to the eNB. If the UE is not downlink synchronized with this cell, then a process similar to cell search has to be performed. As shown in FIG. 1, downlink delay is acquired by way of downlink synchronization. Uplink synchronization is acquired by performing a random access process (at the same time the sending time advance (abbreviated as TA) is acquired), and TA includes uplink and downlink sending delay. TA is mainly used for determining the data sending time by the UE. Since in the LTE system, there is only one carrier in the cell, the downlink of this carrier is taken as the time reference and the time for sending uplink data (time T2 in FIG. 1) is determined in conjunction with TA. T1/T2/T3 are all absolute times, and the E-UTRAN Node B and the UE are aligned. If the UE expects that the eNB receives data at T3, then the UE has to send the data to the E-UTRAN Node B at time T2, and from the view of E-UTRAN Node B, time T2 of UE is equivalent to time T1 of the E-UTRAN Node B. The random access process includes two classes: conflict access process and no conflict access process. The uplink and downlink carriers performing a non-conflict random access process can be paired carriers with the same frequency and can also be paired carriers with different frequencies. The uplink and downlink carriers performing a conflict random access process can only be paired carriers with the same frequency. When performing the non-conflict random access process, the terminal sends a random access preamble to the E-UTRAN Node B (the component carrier sending the random access preamble is the uplink carrier which performs the random access process), and the E-UTRAN Node B returns a random access response to the terminal (the component carrier used by the E-UTRAN Node B to send the random access response is the downlink carrier which performs the random access process). When performing the conflict random access process, as shown in FIG. 2, in addition to carrying out the interaction of the above two messages, the terminal and the E-UTRAN Node B further need to carry out the interaction of the following two messages: the terminal sends a scheduling transmission message to the eNB, and the eNB returns a conflict resolution message to the terminal, for resolving the above conflict.
In order to provide higher data rate to the mobile user, the LTE-Advanced (abbreviated as LTE-A) proposes the carrier aggregation technology, and its objects are to provide larger bandwidth for the user equipment with corresponding ability and improve the peak rate of the UE. In the LTE, the maximum downlink transmission bandwidth supported by the system is 20 MHz, and the carrier aggregation refers to aggregate two or more component carriers (abbreviated as CC) to support a downlink transmission bandwidth greater than 20 MHz and no more than 100 MHz.
The component carrier can use bands already defined by the LTE and can also use dedicated bands newly added for the LTE-A. Since currently there is a lack of spectrum resources and there can not always be component carriers continuous in the frequency domain allocated to the operators for use, the carrier aggregation can be divided into continuous carrier aggregation and non-continuous carrier aggregation according to whether component carriers are continuous in the frequency domain. The carrier aggregation can be divided into single band carrier aggregation and multiple frequency band carrier aggregation based on whether the component carriers are within the same frequency band. The so-called single band carrier aggregation refers to that all the component carriers participating in carrier aggregation are within the same band, and the single band carrier aggregation can be continuous carrier aggregation or non-continuous carrier aggregation. The so-called multiple frequency band carrier aggregation refers to that component carriers participating in carrier aggregation can be from different bands. LTE UE can only receive and transmit data on a LTE compatible component carrier, and an LTE-A UE with carrier aggregation capability (for the sake of description, the UE in the following is such UE, only when specially stated) receive and transmit data on a plurality of component carriers simultaneously. As a contrast, the sending device and receiving device of the UE can be a set of baseband devices, with one single band and the bandwidth thereof being greater than 20 MHz, and can also be multiple sets of baseband devices, with multiple bands and the bandwidth of each band being less than 20 MHz.
In the LTE-A system, after having entered the connection state, the UE can communicate with the source eNB via a plurality of component carriers (such as CC1 and CC2) simultaneously, wherein CC1 is a primary component carrier (abbreviated as PCC), and the other such as CC2 is a secondary component carrier (abbreviated as SCC). The terminal acquires information about the Non-Access Stratum (abbreviated as NAS) (such as Evolved Cell Global Identity (abbreviated as ECGI) and Tracking Area Identity (abbreviated as TAI)) via the primary component carrier, if a radio link failure (abbreviated as RLF) occurs in the downlink primary component carrier, then the UE has to perform a radio resource control (abbreviated as RRC) reestablishment process. After the UE has entered into the connection state from the idle state, the accessing component carrier is the primary component carrier. When the UE is in the connection state, the network can complete the conversion of PCC by way of RRC reconfiguration or intra-cell handover, or the network side designates the primary component carrier during the process of notifying the UE to carry out handover.
In addition, in order to improve the coverage of the eNB, as to some downlink carriers, such as CC2, the Remote Radio Head (abbreviated as RRH) technology can be employed, or a signal repeater can be provided, so that the eNB sends data to the UE on CC1 and CC2 simultaneously, and the time for the data of these two carriers to arrive at the UE will be different, as shown in FIG. 3. If both the uplink and downlink CC2s employ RRH or signal repeater, the eNB only configures CC2 for the UE, and there also exists the same problem. Since the uplink data sending time delays of CC1 and CC2 are the same, it is rational to maintain one TA, however, TA is a relative time quantity, and currently there is still no solution for determining the absolute time for sending uplink data according to this relative time amount.